1. Field of the Invention
The invention relates to demand defrost systems for use with refrigeration units, particularly heat pumps used in both a heating and cooling mode.
2. Description of the Prior Art
Heat pump systems designed to both heat and cool an enclosed space are well known in the prior art. Briefly, these systems consist of a compressor connected by means of a reversing valve between exterior and interior coils which serve as the evaporator and condenser, respectively, when the system is operating in the heating cycle. Heat is exchanged between the refrigerant in the coils and the stream of air blown over each by separate fans.
The exterior coil extracts heat from the exterior air and the interior coil gives it up to the interior air blown over it, both coils thus serving as heat exchangers using a conventional refrigerant as the medium. This heat exchange cycle is reversed when the heat pump is operated in a cooling mode. Typical of such heat pump systems are those shown and described in U.S. Pat. Nos. 4,007,603, 3,950,962, 3,024,722, 3,918,268, 3,400,553, 3,461,681, 3,466,888, 3,529,659, 3,681,933, 3,453,837, and 4,027,497 (copies of which are filed herewith).
As shown in the noted patents, the need to sense and remove frost from the exterior coils (heat source side) when the heat pump is operated in the heating mode to improve heat transfer from the exterior air to the refrigerant has long been recognized. This is particularly troublesome when such heat pump systems are operated in geographical locations where the exterior air temperature is frequently below freezing and the humidity is relatively high.
A variety of approaches have been taken to solve the problem of efficiency robbing frost buildup on the exterior coils. One, as shown in U.S. Pat. No. 3,529,659, simply stops the compressor when there is insufficient exterior radiant heat to prevent frost buildup. An electrical heater is used to replace the interior heat lost by stopping the heat pump.
In U.S. Pat. No. 4,027,497 it is proposed that the refrigerant be heated just prior to entry into the exterior coil to prevent frost buildup on the coil. In U.S. Pat. No. 3,918,268 an auxiliary outdoor coil is used with the exterior coil to prevent the exterior surface subject to frost buildup from falling below freezing.
Timed defrost cycles also have been employed; and while generally effective, such systems are frequently initiated when unneeded, a distinct disadvantage when defrosting is accomplished by using the hot gaseous refrigerant, thus necessitating the unnecessary shutting down of the heating system, a real energy expensive annoyance in cold weather. To avoid this, so-called demand defrost systems have been devised using a number of different methods of initiating the defrost cycle when the frost on the exterior coil has built up to the point where it materially lessens the transfer of heat from the exterior air into the refrigerant.
For instance, U.S. Pat. No. 3,453,837 discloses the concept of initiating a defrost cycle upon the occurrence of a predetermined difference in the temperature of air entering and leaving the condenser. The defrost cycle is terminated when the evaporator coil temperature reaches a determined value.
U.S. Pat. Nos. 3,950,962; 3,681,933, 3,466,888; 3,777,505; 3,400,553 and 4,024,722 disclose the use of temperature sensors, singly or in combination, to initiate and terminate the defrost cycle. As observed in U.S. Pat. No. 3,950,962, a disadvantage of so-called "temperature-difference" defrost systems is that strong, continuous or gusty winds can effect the exterior temperature to be sensed. This has frequently been found to effect both the proper initiation and termination of the defrost cycle.
U.S. Pat. Nos. 3,461,681 and 4,007,603 disclose yet other demand defrost systems that include means for sensing and utilizing both temperature and the pressure differentials across the exterior coil to initiate and terminate the defrost cycle. These pressure-difference systems are also subject to faulty operations caused by strong gusts of wind.
A commercially available demand defrost control that initiates a defrost cycle on sensing a pressure differential through an evaporator coil and terminates upon sensing a determined temperature of the coil is the Model DS 10 Series Demand Defrost Control manufactured by the Robertshaw Controls Company, 1701 Byrd Avenue, the assignee of this application.